This invention relates to an acid concentration measuring apparatus and an acid concentration measuring method which can measure for a sufficiently short period of time and accordingly at sufficiently short measurement intervals the acid concentration of a pickling solution contained in a pickling tank of a continuous pickling line for steel stock, etc. It also relates to an acid concentration automatic control apparatus which, using the acid concentration measuring apparatus, can automatically control the acid concentration of a pickling solution contained in a pickling tank, such as an acid concentration automatic control apparatus and an acid concentration automatic control method which are suitable for use with a continuous pickling line of the type in which pickling solution contained in a pickling tank on a downstream side is made to successively overflow to adjoining pickling tanks on an upstream side.
Pickling is a treatment process in which an oxide scale present on the surface of steel plates, such as cold rolled steel plates, hot rolled steel plates to be subjected to cold rolling, hot rolled steel plates which are in the form of final products, is removed by immersing the steel plates in a pickling solution of hydrochloric acid, sulfuric acid, etc., or by spraying a pickling solution at the surface of the steel plates. Pickling is performed, for example, by continuously passing steel plates through a continuous pickling line in which a plurality of pickling tanks containing a pickling solution are arranged in series in the direction of movement of the plates. In pickling performed using such a continuous pickling line, the acid concentration in each pickling tank, and particularly in the last pickling tank, greatly influences the efficiency of removal of oxide scale. Therefore, in pickling using such a continuous pickling line, it is necessary to accurately control the acid concentration.
In the past, in a continuous pickling line for steel strip, acid concentration measuring of pickling solution contained in a pickling tank and the supply of acid solution were performed by measuring the acid concentration of pickling solution contained in a pickling tank using a table-top measuring device and manually supplying acid solution to the pickling tank based on the results of measuring, or by installing the table-top measuring device on a pickling tank of the continuous pickling line to automatically measure acid concentration and automatically control the amount of acid solution supplied to the pickling tank based on the results of measuring.
However, when acid solution is manually supplied to a pickling tank, it is not possible to accurately cope with changes in the concentration of the pickling solution in the pickling tank. Therefore, it is easy for fluctuations in the concentration of the pickling solution to become large, and it is easy for the amount of acid solution which is supplied to become too great when trying to stay on the safe side. Therefore, the consumption of the pickling solution per unit of output tends to become poor when acid solution is manually supplied to a pickling tank.
A titration-type analyzer is used when automatically measuring the acid concentration of pickling solution in a pickling tank with a table-top measuring apparatus installed on the pickling tank. Measuring using a titration-type analyzer is performed by successively introducing a sample liquid, a reagent, and a pickling solution into a measuring cell. Therefore, the sample liquid flows intermittently into the measuring cell, so the retained sample liquid solidifies in piping and clogs up the piping, and measuring becomes impossible a short time after it is begun.
In addition, when performing measuring using a titration-type analyzer, only a minute quantity of sample liquid is supplied, so fine tubes within the piping system become clogged. If a filtering apparatus is provided to prevent clogging, a complicated piping system having a switching mechanism becomes necessary. An increase in the number of times the switching mechanism is switched leads to clogging of the piping.
In addition, a titration-type analyzer is expensive. Therefore, when a plurality of types of sample liquid are present, the sampling pipes much be connected in parallel to a single titration-type analyzer, and measuring is carried out by switching each sampling pipe. The switching of the sample pipes also causes clogging of the pipes to become frequent.
Furthermore, when measuring with a titration-type analyzer, approximately 15 minutes are required for one sampling from the introduction of the sample liquid until the output of data. Therefore, when sampling is carried out more than once using the same titration-type analyzer, the interval between data outputs is considerable, i.e., a minimum of 15 minutes. As a result, even if a titration-type analyzer is applied to an acid concentration control system for a continuous pickling line, it is actually impossible to output the measured acid concentration of the pickling solution at sufficiently short measurement intervals.
Thus, measurement of acid concentration using a titration-type analyzer has a long reaction time for chemicals, a long switching time by a pretreatment apparatus at the time of cleaning, and a long sampling time. Therefore, a time lag between sampling and measuring is unavoidable. Furthermore, when measuring acid concentration using a titration-type analyzer, data is output intermittently at considerable intervals. Thus, the responsiveness of control is extremely poor. Accordingly, it was difficult to control the acid concentration of a pickling solution using a titration-type analyzer with high accuracy.
In this manner, measuring the acid concentration of a pickling solution in a pickling tank requires much time. Therefore, particularly in a continuous pickling line of the type in which the pickling tanks are separated by partitions and pickling solution in a pickling tank on a downstream side successively overflows to pickling tanks on an upstream side and acid solution is supplied to the final pickling tank, normally the acid concentration of only the final pickling tank is measured to determine the amount of acid solution to be supplied, whereby the supply of acid solution is controlled in accordance with the acid concentration of the pickling solution in the final pickling tank.
However, in this type of continuous pickling line, reaction between the pickling solution and the scale layer on steel strip is actually more vigorous in more upstream pickling tanks than in the final pickling tank. Therefore, when fluctuation in the acid concentration of the pickling solution is grater in the upstream pickling tanks than in the final pickling tank, scale may remain on the steel strip.
If scale remains on steel strip, it becomes necessary to stabilize the acid concentration of the pickling solution in each pickling tank by lowering the line speed. In addition, in order to prevent scale from remaining, it is necessary to control the acid concentration of pickling solution in each pickling tank more rigorously than normally. Therefore, even if it is possible to measure the acid concentration and control the supply of acid solution in real time, increases in acid consumption per unit of output, i.e., in cost are unavoidable.
Therefore, in the past, various proposals have been made of ways to speedily control the acid concentration of a pickling solution with high responsiveness to compensate for the fact that the acid concentration of pickling solution in a pickling tank could not be measured as it continuously flowed.
For example, Japanese Published Examined Patent Application Sho 57-2275 discloses an invention in which, because of the fact that in feedback control of the acid concentration of pickling solution, hatching takes place if the gain is made large while the accuracy of a sensor decreases if the gain is made small and the sensor can""t be used, instead of feedback control, feed-forward control is carried out using a formula relating liquid temperature, acid concentration, acid reaction time, and the reactive surface area to improve the responsiveness of control of the acid concentration of a pickling solution.
Japanese Published Unexamined Patent Application Hei 6-126322 discloses an invention for use in a spray-type pickling line in which the acid concentration of the pickling solution in a most upstream circulation tank is adjusted and acid solution in an amount corresponding to the amount of acid charged at that time is supplied to a circulation tank just downstream thereof, and this adjustment of acid concentration and supply of acid is successively carried out with respect to downstream circulation tanks, whereby the acid concentration of the pickling solution in each circulation tank is controlled.
Japanese Published Unexamined Patent Application Hei 9-125270 discloses an invention in which a circulation tank is connected to a pickling tank by piping and pickling solution is circulated. Pickling solution is removed from a portion of the piping, and the acid concentration of the removed pickling solution is continuously measured using an acid concentration analyzer. Furthermore, the surface level of pickling solution in a circulation tank is measured, and when the measured value deviates from a target value, discharge of acid, addition of acid, or addition of water is carried out.
Japanese Published Unexamined Patent Application Hei 10-147895 discloses a method for controlling the acid concentration of pickling solution in a pickling tank in a continuous pickling line of the type having a separate circulation apparatus for pickling solution provided for each pickling tank, with no overflow of pickling solution between adjoining pickling tanks.
Japanese Published Unexamined Patent Application Hei 7-54175 discloses a method in which the weight loss due to pickling is determined from the difference in the plate thickness of a steel plate between before and after pickling in a continuous pickling line, and the supply of acid and the acid concentration are controlled based on the thus determined weight loss due to pickling.
However, in the invention disclosed in Japanese Published Examined Patent Application Sho 57-2275, the required measuring of acid concentration is not performed while the pickling solution is continuously flowing. Therefore, in that invention, it is not possible to control the acid concentration with high accuracy. Furthermore, when carrying out that invention, if acid concentration is measured over a long period of time, sampling piping becomes clogged due to acid, leading to a decrease in the operating efficiency of the measuring device.
When continuously controlling acid concentration by the invention disclosed in Japanese Published Unexamined Patent Application Hei 6-126322, it is necessary to measure the acid concentration of the pickling solution in a final circulation tank at sufficiently short measurement intervals. However, as previously stated, it is impossible to measure the acid concentration of pickling solution at sufficiently short measurement intervals. Therefore, that invention cannot control the acid concentration of a pickling solution with high accuracy either.
The acid concentration analyzer used in the invention disclosed in Japanese Published Unexamined Patent Application Hei 9-125270 is installed through a valve on a branch pipe which branches from a return pipe for pickling solution. Therefore, it can be seen that this acid concentration analyzer is of the same type as the above-described titration-type analyzer. Thus, in that invention as well, it is not possible to measure the acid concentration of pickling solution at sufficiently short measurement intervals. As a result, in that invention as well, it is difficult to control the acid concentration of pickling solution with high accuracy. Furthermore, in that invention, when water is added, the acid concentration of waste acid decreases, so the acid consumption per unit of output at the time of recovery of waste acid becomes poor.
In the continuous pickling line disclosed in Japanese Published Unexamined Patent Application Hei 10-147895, it is possible to independently control the acid concentration of the pickling solution in each pickling tank, so it is possible to control the acid concentration of pickling solution with high accuracy. However, control according to that proposal cannot be applied without any equipment modification to a continuous pickling line of the type in which pickling solution overflows between adjoining pickling tanks. Namely, in order to apply the method disclosed in Japanese Published Unexamined Patent Application Hei 10-147895 to a continuous pickling line in which pickling solution is made to overflow between adjoining pickling tanks, it is necessary to install a circulation tank for acid solution, a circulation pump, piping for waste acid and for the supply of acid, etc., for each pickling tank. Therefore, considerable investment in equipment and installation space are required, so it is actually quite difficult to carry out that invention.
Furthermore, scale loss during pickling varies with the scale thickness, which varies depending, for example, on the winding temperature during hot rolling. Therefore, in the invention disclosed in Japanese Published Unexamined Patent Application Hei 7-54175, the weight loss during pickling and the change in the acid concentration of the pickling solution are not necessarily equal to each other. For this reason, the accuracy of control of the acid concentration of a pickling solution is decreased to the extent that a deviation develops between the change in the acid concentration of the pickling solution and the weight loss due to pickling.
Thus, all of the methods of the prior art have the fatal problemxe2x80x94that the acid concentration of pickling solution contained in pickling tanks cannot be measured at sufficiently short measurement intervals. Therefore, not only when the control of the supply of acid solution is performed manually but also when it is performed automatically, it was necessary to put up with response lags and a decrease in accuracy in the control of the acid concentration of pickling solution.
U.S. Pat. No. 5,175,502 proposes an invention in which pickling solution removed from a pickling tank is diluted with water to prevent clogging by the pickling solution, and the acid concentration of the pickling solution is measured based on the density, the conductivity, and the temperature of the diluted pickling solution. However, in that invention, pickling solution is diluted with water, so the pickling solution ends up being waste liquid after measuring. For this reason, costs end up increasing with that invention.
The object of the present invention is to provide an acid concentration measuring apparatus and an acid concentration measuring method which can measure the acid concentration of pickling solution in a pickling tank at sufficiently short measurement intervals, as well as an acid concentration automatic control apparatus and an acid concentration automatic control method which use the acid concentration measuring apparatus to automatically control the acid concentration of pickling solution supplied to each of the pickling tanks making up a continuous pickling line with sufficiently short measurement intervals and with high accuracy.
Specifically, the object of the present invention is to provide an acid concentration measuring apparatus and an acid concentration measuring method which, by performing continuous measuring of a sample of a pickling solution in a pickling tank at sufficiently short measurement intervals, can shorten the periods of time in which measuring is not taking place and can substantially continuously and accurately measure variations and segregation in the acid concentration of the pickling solution and which has a simple sampling method for pickling solution and is excellent with respect to ease of maintenance, as well as an acid concentration automatic control apparatus and an acid concentration automatic control method which, using this acid concentration measuring apparatus, can automatically control the acid concentration of pickling solution in each pickling tank making up a continuous pickling line substantially continuously and with high accuracy.
More particularly, the object of the present invention is to provide an acid concentration automatic control apparatus and an acid concentration automatic control method which, using the above-described acid concentration measuring apparatus, measures at sufficiently short measurement intervals the acid concentration of pickling solution in a pickling tank in a continuous pickling line of the type in which pickling solution in a pickling tank on a downstream side is made to successively overflow to adjoining pickling tanks on an upstream side and the results of measuring are fed back to the supplied amount of acid solution, whereby the acid concentration of the pickling solution in each pickling tank can be maintained at a proper level and the consumption of pickling solution per unit of output can be improved.
The present inventors performed various investigations aimed at accomplishing these objectives. They fixed their attention on causing pickling solution in pickling tanks to continuously flow and to measuring the acid concentration of the pickling solution as it is continuously flowing. In the past, even if pickling solution was made to continuously flow in this manner, regions where the flow speed decreased always developed, and it was thought that the pickling solution would clog up in a short length of time in the regions of decreased flow speed. Therefore, up to the present time, the measuring of the acid concentration of pickling solution as it is continuously flowing was not investigated.
As a result, the present inventors found that by installing a measuring device which continuously measures a physical property other than the acid concentration of continuously flowing pickling solution in a flow path of the pickling solution and by performing calculations based on the measured data output by the measuring device, clogging by the pickling solution can be virtually eliminated, and the acid concentration of pickling solution in the pickling tanks can be determined substantially continuously as well as accurately at sufficiently short measurement intervals.
The present inventors also found that by performing feedback control or feedback control and feed forward control of the acid concentration of pickling solution in a pickling tank to which acid solution is supplied based on the calculated acid concentration obtained in the above manner, it is possible to rapidly and precisely cope with constant variations in acid concentration and to control with high accuracy the acid concentration of pickling solution in a pickling tank.
The present inventors additionally found that by performing feedback control of the acid concentration of pickling solution in a pickling tank to which acid solution is supplied based on the calculated acid concentration in a pickling tank to which acid solution is supplied and in at least one pickling tank other than a pickling tank to which acid solution is supplied, it is possible to control with high accuracy the acid concentration of pickling solution in a pickling tank, even in a continuous pickling line of the type in which pickling solution in a pickling tank on a downstream side is made to successively overflow to adjoining pickling tanks on an upstream side.
Based on these findings, the present inventors performed further investigations and completed the present invention.
The gist of the present invention is an acid concentration measuring apparatus characterized by being equipped with the combination of a body which is installed on a portion of a flow path of pickling solution which is taken from a pickling tank, the flow path being connected to the pickling tank, a density sensor which measures the density of the pickling solution flowing within the body, a temperature sensor which measures the temperature of the pickling solution in the flow path or the pickling tank, a conductivity sensor which measures the conductivity of the pickling solution in the flow path or the pickling tank, and a calculating apparatus which calculates the acid concentration of the pickling solution flowing through the portion of the flow path based on the measurements by the density sensor, the temperature sensor, and the conductivity sensor. This will be referred to below as xe2x80x9cthe first inventionxe2x80x9d.
In the above-described first invention, one or both of the temperature sensor and the conductivity sensor is preferably installed on the body.
In the acid concentration measuring apparatus according to the first invention, the density sensor is preferably a differential pressure type density sensor having at least two sensing portions. In this case, the two sensing portions are preferably installed at least 500 mm apart in the direction in which a flow path is formed in the body in order to maintain a desired measurement accuracy.
Also, in the acid concentration measuring apparatus according to the first invention, the temperature sensor and the conductivity sensor are preferably both installed on the exit side of the flow path in the body in order to maintain a desired measuring accuracy.
Furthermore, in the acid concentration measuring apparatus according to the first invention, the flow path within the body is preferably made as straight as possible so as not to form local regions where the flow speed decreases, and in regions of the flow path in which the flow speed decreases and it is easy for pickling solution to clog, a clogging prevention mechanism which suppresses the lingering of pickling solution is preferably installed in order to prevent clogging by the pickling solution.
From another standpoint, the present invention is an acid concentration measuring method characterized by measuring the density of pickling solution continuously flowing within the interior of an acid concentration measuring apparatus body which is installed in a portion of a flow path of pickling solution taken from a pickling tank, the flow path being connected to the pickling tank, measuring the temperature and the conductivity of the pickling solution in the flow path or the pickling tank, and calculating the acid concentration of the pickling solution flowing in the portion of the flow path based on the results of measurement of the density, the temperature, and the conductivity. This will be referred to below as xe2x80x9cthe second inventionxe2x80x9d.
From another standpoint, the present invention is an acid concentration automatic control apparatus characterized by being equipped with the combination of the acid concentration measuring apparatus according to the first invention which is installed on the pickling tank to which acid solution is supplied out of a plurality of pickling tanks forming a continuous pickling line, and a feedback control means which adjusts the acid concentration of pickling solution in the pickling tank to which acid solution is supplied based on the calculated value of acid concentration obtained by the acid concentration measuring apparatus. This will be referred to below as xe2x80x9cthe third inventionxe2x80x9d.
From another standpoint, the present invention is an acid concentration automatic control apparatus characterized by being equipped with the combination of the acid concentration measuring apparatus according to the first invention which is installed on a pickling tank to which acid solution is supplied and on at least one pickling tank other than a pickling tank to which acid solution is supplied of a plurality of pickling tanks forming a continuous pickling line, and a feedback control means which adjusts the acid concentration of pickling solution in a pickling tank to which acid solution is supplied based on the calculated value of acid concentration obtained from each of a plurality of acid concentration measuring apparatuses. This will be referred to below as xe2x80x9cthe fourth inventionxe2x80x9d.
In the acid concentration automatic control apparatus according to the third invention or the fourth invention, a feed forward control means is preferably provided to adjust the acid concentration of pickling solution in a pickling tank to which acid solution is supplied based on the calculated value of acid concentration obtained from the acid concentration measuring apparatus in order to further increase the responsiveness of control. This will be referred to below as xe2x80x9cthe fifth inventionxe2x80x9d.
The pickling tanks which are the objects being controlled by the acid concentration measuring apparatus, the acid concentration automatic control measuring method, or the acid concentration automatic control apparatus according to the first through fifth inventions are not limited to being immersion types but may instead be spray types.
In the acid concentration automatic control apparatuses according to the third through fifth inventions, the pickling tank to which acid solution is supplied is shown, by way of example, as being the final pickling tank.
Furthermore, the continuous pickling line to which the acid concentration automatic control apparatus according to the third through fifth inventions is applied is illustrated, by way of example, as a continuous pickling line of the type in which pickling solution in a pickling tank on a downstream side is made to successively overflow to adjoining pickling tanks on an upstream side.
From another standpoint, the present invention is an acid concentration automatic control method characterized in that an acid concentration measuring apparatus according to the first invention is installed at least on a pickling tank to which acid solution is supplied out of a plurality of pickling tanks forming a continuous pickling line, and based on a calculated value of acid concentration obtained from this acid concentration measuring apparatus, feedback control or a combination of feedback control and feed forward control is carried out to adjust the acid concentration of pickling solution in the pickling tank to which acid solution is supplied. This will be referred to below as xe2x80x9cthe sixth inventionxe2x80x9d.
According to the acid concentration measuring apparatus of the first invention or the acid concentration measuring method of the second invention, the density, the temperature, and the conductivity of a pickling solution can be measured at sufficiently short measurement intervals, so the acid concentration of pickling solution can be measured over a long period substantially continuously at sufficiently short measurement intervals. Furthermore, a construction is employed such that pickling solution does not clog in flow passages within which pickling solution is flowing, so the ease of maintenance is improved. Accordingly, an acid concentration measuring apparatus according to the present invention makes it possible to perform measurements over long periods.
According to the acid concentration automatic control apparatus of the third through fifth inventions or the acid concentration automatic control method of the sixth invention employing this acid concentration measuring apparatus, the acid concentration of pickling solution in a pickling tank to which acid is supplied can be controlled with high accuracy and stability. As a result, acid consumption per unit of output is improved.
In particular, according to the acid concentration automatic control apparatus of the fifth invention, a combination of continuous measuring of acid concentration using the acid concentration measuring apparatus according to the first invention is combined with feedback control of the acid concentration, and on this foundation is superimposed feed forward control of acid concentration. Therefore, according to the acid concentration automatic control apparatus of the fifth invention, the accuracy and responsiveness of control of the acid concentration of pickling solution in a pickling tank can both be enormously improved.
Furthermore, in the acid concentration automatic control apparatus according to the third through fifth inventions, using the acid concentration measuring apparatus according to the first invention, the acid concentration of pickling solution in each of the pickling tanks in a continuous pickling line of the type in which pickling solution in a pickling tank on a downstream side is made to successively overflow to adjoining pickling tanks on an upstream side is measured at sufficiently short measurement intervals, and the measured result is fed back to the amount of acid solution which is supplied. Therefore, the acid concentration of the pickling solution in each pickling tank can be maintained at an appropriate level, and the consumption of pickling solution per unit of output can be decreased.